Catalysts oxazolidone

Base-catalyzed Diels-Alder reactions are rare (Section 1.4). A recent example is the reaction of 3-hydroxy-2-pyrone (145) with chiral N-acryloyl oxazolidones 146 that uses cinchona alkaloid as an optically active base catalyst [97] (Table 4.25). Only endo adducts were obtained with the more reactive dienophile 146 (R = H), the best diastereoselectivity and yields being obtained with an i-Pr0H/H20 ratio of 95 5. The reaction of 146 (R = Me) is very slow, and a good adduct yield was only obtained when the reaction was carried out in bulky alcohols such as t-amyl alcohol and t-butanol. 

An alternative sequence utilized 2-oxazolidone, which was readily synthesized from urea and ethanolamine, as the glycine equivalent. Subsequent treatment with phosphorous acid and formaldehyde produced iV-phosphonomethyl-2-oxazolidone 12 (16). Upon hydrolysis, and loss of CO2,12 provided the related derivative, iV-phosphonomethylethanolamine 13, which was oxidized at high temperature with a variety of metal catalysts including cadmium oxide (16) or Raney copper (17) to give GLYH3, after acidification. A similar oxidation route has also been reported starting from iV-phosphonomethy 1-morpholine (18). 

A comprehensive review of trimerization catalysts was prepared by Zhitankina et al (116). Isocyanate trimerization catalysts are shown in Table 11. 2-Oxazolidone catalysts and carbodiimide catalysts are shown in Tables 12 and 13 respectively. 

The synthesis and properties of 2-oxazolidones and 2-oxazolidone-containing polymers have been reviewed by Pankroatov et al (115). It is interesting to note that some isocyanate cyclotrimerization catalysts also act as catalysts for urethane, oxazolidone and/or carbodiimide linkages. 

Cyclization of 2-butene dicarbamate (123) in the presence of a palladium catalyst coordinated with chiral ligands (124) gave optically active 4-vinyl-2-oxazolidones (125), which were hydrolyzed to optically active 2-amino-3-butenols (Scheme 54). ° ... 

The reaction of -NCO isocyanate groups with epoxidic rings, in the presence of special catalysts, leads to the formation of cyclic urethanes (oxazolidones) [1, 3, 12, 13, 23-25] ... 

By using sodium or potassium hydroxides as catalysts of aminolysis, it is possible to regenerate the initial monoethanolamine (a bifunctional compound in the conditions of PU chemistry and to destroy the oxazolidone formed (reaction 20.11) [33] ... 

When an NC0 epoxy ratio of 2 1 was used, an NCO-terminated, oxazolidone-containing prepolymer was formed that was then trimerized by employing a trimerization catalyst to yield poly(oxazolidone-isocyanurates) as shown below ... 

Epoxy-terminated oxazolidone prepolymers were prepared by using a ratio of NCO epoxide of 1 2. These prepolymers could then be cured with conventional epoxy hardeners or catalysts ... 

However, these results could not be generalized for intermolecular [4 - - 2] cycloadditions (Scheme 7.6) [9]. 25 was formed in 38 % ee at normal pressure from isoprene (23) and the oxazolidone 24, while the enantioselectivity decreased at 5 kbar to only 21 % ee. Nevertheless, this study revealed another important factor that has to be taken into account for metal-catalyzed reactions under pressure. It was argued that the decreased selectivity may in part be due to a shift of the chiral catalyst 28 to the achiral catalyst precursor 26 induced by pressure, since this... 

Experi- ment No. Epoxide (moles) Isocyanates (moles) Catalyst (gm) Solvent Reaction conditions 2-Oxazolidone m.p. (°C) Yield (%) Characteristic infrared spectral bands and comments Ot)... 

Epoxy Resin Isocyanate Q Catalyst Solvent Reaction Reaction Oxazolidone Spt., C Yield Characteristic IR... 

The reaction of the isocyanate and the epoxide group, to produce an oxazolidone ring, was studied through the model reaction between the monofunctional epoxide, PGE and the bifunctional isocyanate MDI. Their reaction was followed by DSC. Two catalysts for the reaction were evaluated tetraethyl ammonium bromide (R NBr) and ethylmethylimidazole (EMI). 

A similar picture emerged when EMI was substituted for R NBr as catalyst. This replacement altered the thermograms in detail but not in the gross features thus, twin exothermic peaks having the same origins as previously determined were observed. However, both peaks were displaced to lower temperatures and the size of the peaks altered with the area of the oxazolidone peak increased between three and fourfold (Table 6) when compared to the oxazolidone peak observed in R NBR catalysed reactions. 

Catalyst From Peak 1, Fig. 1 Trimerisation of isocyanate to isocyanurate (IV) From Peak 2, Fig. 1 Oxazolidone formation (I)... 

DSC Study of Oxazolidone Containing Resins. The DSC results of the PGE/ MDI model reaction, using R NBr or EMI as catalyst, showed that both promoted the trimerisation and the oxazolidone formation, with the latter catalyst being more powerful in respect of each reaction. The gained information suggested that by a careful choice of catalyst, catalyst concentration, and... 

The formation of oxazolidone was followed by DSC using BADGE and MDI and the three catalysts, i.e., EMI, DABCO, and HEXA. By a comparison of thermograms some distinctions between the PGE/MDI and the BADGE/MDI reactions emerged. 

Catalyst Trimerisation peak temperature C Oxazolidone peak temperature °C ... 

Catalyst BADGE Moles Relative oxazolidone concentration (from 1740 cm absorption) Relative isocyanurate concentration (from 1710 cm absorption)... 

Table 11 compares the peak heights for oxazolidone and isocyanurate absorptions, with these frequencies normalised to the absorption at 1510 cm From these results, we concluded hat HEXA had the greatest efficiency as an oxazolidone forming catalyst under the conditions of the film casting procedure. In contrast, the DABCO catalysed resin had the strongest isocyanurate... 

The reaction temperature has a measurable influence on the oxazolidone formation when HEXA is used as catalyst, this point is illustrated in Table 12, oxazolidone concentration increasing with temperature increase. 

Second, for the preparation of ISOX resins, the preliminary finding is that EMI, being an effective catalyst for the formation of both oxazolidone and isocyanurate, is the preferred catalyst. This conclusion is based on finding that DABCO is a strong catalyst for isocyanurate formation and HEXA a good catalyst for oxazolidone formation. This latter conclusion is to be regarded as preliminary. 

Phosphorylation of Alcohols with A Phosphoryloxazolidi-nones. This reaction can be efficiently performed by using copper(II) triflate as catalyst, and the conditions are milder than those errqiloyed with alkoxides. Phosphoryl transfer from N-phosphoryl 5,5-diphenyl oxazolidone is an important process in S3mthesis of biomolecules. The optimal ligand is A(,A -ethy lenebis-(benzaldimine)(BEN) with 0.2 equiv of Cu(OTf)2 as the best catalyst (eq 37). The ligand-bound Cu species activates the P=0 bond for nucleophilic attack by the alcohol. 

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